Optical recording has been increasingly used in recent years to publish, distribute, store, and retrieve digital information. This is done by focusing a laser beam to write and/or read information on an optical recording media usually in the form of a spinning disk. In the read only memory (ROM) format, the information is prefabricated at the factory in the form of encoded small features on the disk and the laser beam is used to read back the information. In the writable formats, the laser beam is used to create the small encoded marks through a variety of physical recording mechanisms. This permits the user to record their own data on the disk. Some recording physical mechanisms are reversible. The recorded marks can be erased and remade repeatedly. Disks that utilize these mechanisms are called Erasable or Re-writable disks. Some of these physical mechanisms are one way, once the marks are made they cannot be reversed or altered without leaving a clearly identifiable trace that can be detected. Disks that utilize these mechanisms are called WORM disks. Each of these formats is suitable for certain practical applications.
The present invention focuses on WORM disks. The popularity of compact disk recordable (CD-R), which is a WORM disk, in recent years suggests that the demand of WORM disks is very strong. This format is suitable for many applications where the data are to be stored in such a form that any modification to the content is not possible without leaving a easily detectable trace. The format is also suitable for many applications where rewriteability is not needed such as those for publishing and distribution. In these applications the WORM disks are desirable if they can be provided at lower costs than the rewritable.
Many physical mechanisms have been used for WORM recording. The first practical WORM disks utilized ablative recording where the pulsed laser beam is used to create physical pits in the recording layer. This mechanism requires the disks to be in an air-sandwiched structure to leave the surface of the recording layer free from any physical obstruction during the pit formation process. The other mechanism is to use the laser beam to cause the fusing or chemical interaction of several layers into a different layer. This mechanism suffers from the requirement of relatively high laser power and from the difficulty in manufacturing the multi-layered structure. The other mechanism is to use organic dye as the recording layer. Although used successfully in CD-R disks, this mechanism suffers from its strong wavelength dependence. The optical head used in the DVD devices operating at 650 nm, for example, is not able to read the CD-R disks designed to work at the CD wavelength of 780 nm. Yet another mechanism is to use phase-change recording media. In contrast to the more popular erasable phase-change media, these WORM media are designed to record data via laser crystallization of the as-deposited amorphous phase. The materials are designed such that it is practically impossible to reverse the laser crystallized state back into the amorphous state. Whereas this mechanism is able to minimize or eliminate most of the problems mentioned above, it is unique in its recording polarity. The crystalline recorded marks have higher reflectivity than the amorphous background, opposite to most other recording mechanisms. This makes it difficult to produce multi-function drive devices.